1. Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device capable of minimizing a change in an electrical property of a thin film transistor (TFT) formed on a flexible substrate when the flexible substrate is bent in one or more directions.
2. Discussion of Related Art
Recently, a technology has been developed for forming a thin film transistor (TFT) using a semiconductor layer formed on an insulating substrate. The TFTs are used for electronic devices such as an integrated circuit and an electro-optical device, and more particularly, are used as switching devices and driving devices for organic light emitting display devices. Insulating substrates such as glass and quartz used for forming the above-described TFTs are fragile and heavy such that they cannot be easily made large and thus cannot be easily mass-produced.
In order to solve the above problem, recently, TFTs are formed on a thin substrate, for example, a thin film-shaped metal substrate or a film-shaped plastic substrate. Since the thin film-shaped metal substrate and the film-shaped plastic substrate can be made to be flexible as well as thin and light, it can be used for a cylindrical display device as well as for a conventional display device (or show window).
However, since the film-shaped plastic substrate is vulnerable to heat and is sensitive to change in temperature, when the TFTs are formed on the film-shaped plastic substrate, the TFTs must be manufactured at a low temperature. Therefore, when the TFTs are formed on the film-shaped plastic substrate, the performance of the TFTs may deteriorate more easily as compared with the case in which the TFTs are formed on a glass substrate.
Therefore, in order to solve the above problem, it is suggested that an organic light emitting display device including the TFTs formed on the thin film-shaped metal substrate be formed. The thin film-shaped metal substrate is not completely bendable, however; the thin file-shaped metal substrate is less vulnerable to moisture and oxygen and can undergo high temperature processes. Therefore, when the thin film-shaped metal substrate is used, it is possible to manufacture the TFTs having high channel mobility. Therefore, it is possible to realize a system on panel (SOP) in which a driving circuit is simultaneously integrated with a pixel. Also, since the metal substrate is flexible, it is possible to realize a high resolution and high performance roll type display device.
FIG. 1 schematically illustrates a TFT formed on a flexible substrate. Referring to FIG. 1, a TFT 10 includes a semiconductor layer having a channel region 11, a source electrode 12, and a drain electrode 13 formed on a flexible substrate 1. In FIG. 1, the channel region 11 of the TFT 10 is formed to be parallel to the direction in which the substrate 1 is bent.
However, when a TFT is formed on a flexible substrate as described above, the array (or structure) of the TFT is strained in the direction where the substrate is bent. In general, when stress is applied to the semiconductor layer that forms the TFT, charge mobility and effective charge mass are affected by the stress so that the electrical property of the TFT changes. To be specific, when the direction in which the substrate is bent is parallel to the direction in which the channel of the TFT is formed (that is, the direction in which current flows), a change in a strain-induced drain current increases so that an electrical property of the TFT changes.